\1 better written as $1 at tmp.pl line 5 (#1)
 
    (W) Outside of patterns, backreferences live on as variables.  The use
    of backslashes is grandfathered on the right-hand side of a
    substitution, but stylistically it's better to use the variable form
    because other Perl programmers will expect it, and it works better
    if there are more than 9 backreferences.